The present invention relates to an apparatus for treating a molten metal such as aluminum or magnesium in a molten state, and more particularly to an apparatus for use in treating a molten metal in a treating container by releasing a treating gas into the molten metal in the form of finely divided bubbles and dispersing the bubbles through the entire body of molten metal to remove hydrogen and other dissolved harmful elements and non-metallic inclusions from the molten metal.
The terms "aluminum" and "magnesium" as used herein include pure aluminum and aluminum alloys, and pure magnesium and magnesium alloys, respectively. Further the term "inert gas" as used herein refers to argon gas, helium gas, krypton gas and xenon gas in the Periodic Table and also to other gases which are inert to the metal to be melted, such as ntirogen gas which is inert to aluminum.
Apparatus of the type mentioned heretofore used for treating molten metal comprise a treating container having an opening at the upper end and a closure for openably closing the opening, a vertical rotary shaft extending through the closure into the treating container and having a treating gas channel internally extending through the shaft longitudinally thereof, and a bubble releasing-dispersing rotor attached to the lower end of the rotary shaft and having in its bottom a treating gas outlet in communication with the gas channel of the rotary shaft (see U.S. Pat. No. 4,611,790). The rotary shaft is rotated while supplying a treating gas, such as an inert gas or chlorine gas, to the gas channel of the shaft to release the gas into molten metal within the container in the form of finely divided bubbles and disperse the bubbles through the entire body of molten metal for the treatment of the molten metal.
It appears possible to improve the efficiency of treatment by the conventional apparatus described by finely dividing the injected gas and dispersing the gas through the interior of the container more effectively. The rotor must be rotated at a higher speed to divide the gas more finley and disperse the bubbles uniformly throughout the container. However, if the speed of rotation of the rotor is increased in the conventional apparatus, the surface of the molten metal markedly eddies and waves, consequently promoting formation of an oxide by the contact of the metal with the atmosphere and penetration of hydrogen into the molten metal (e.g. 2Al+3H.sub.2 O.fwdarw.Al.sub.2 O.sub.3 +6H). The eddy aslo incorporates the dross on the surface of the melt into the body of molten metal. Moreover, the vigorous eddy collects bubbles of gas in the center of the eddy, i.e. in the vicinity of the rotary shaft for the rotor, no longer permitting the dispersion of gas bubbles through the entire treating container. Consequently, the treatment efficiency rather lowers.
Accordingly, an apparatus has been proposed which is of the same type as above and in which a baffle for preventing eddying and waving is suspended from the closure covering the open upper end of the container so as to be immersed in the molten metal within the container (see Utility Model Publication No. Sho 60-161162). Nevertheless, when the vertical rotary shaft of the baffle is to be repaired or replaced, there arises a need to upwardly withdraw the shaft from the container and to remove the closure. The proposed apparatus thus requires a cumbersome procedure.
Further when the dross floating on the surface of the molten metal within the container is to be removed after the completion of treatment, the rotary shaft and the baffle become obstacles to this procedure, so that the shaft and the cover must be withdrawn or removed, hence cumbersomeness.